The present invention relates to ablation instrument systems that use electromagnetic energy in microwave frequencies to ablate internal bodily tissues. More particularly, the present invention relates to an antenna arrangement in an ablation instrument system that directs the microwave energy in a selected direction.
Catheter ablation has recently become an important therapy for certain cardiac arrhythmias, cardiac disrhythmias and tachycardia. Many ablation instrument systems utilize radio frequency (RF) energy as the ablating energy source. Accordingly, a variety of RF based catheters and power supplies are currently available to electrophysiologists. However, radio frequency energy has several limitations including the rapid dissipation of energy in surface tissues resulting in shallow "burns" and failure to access deeper arrhythmic tissues. Another limitation of RF ablation instruments is the risk of clot formation on the energy emitting electrodes. Such clots have an associated danger of causing potentially lethal strokes in the event that a clot is dislodged from the ablation instrument.
A second common ablation approach is the use of high voltage, direct current defibrillator discharges. Direct current ablation has several drawbacks including the need for general anesthesia and explosive discharges that can cause debris or even rupture certain cardiac organs. For these and other reasons, significant attention has been given recently to alternative ablative energy sources.
Microwave frequency energy has long been recognized as an effective energy source for heating biological tissues and has seen use in such hyperthermia applications as cancer treatment and preheating of blood prior to infusions. In view of the drawbacks of the traditional catheter ablation techniques, there has recently been a great deal of interest in using microwave energy as an ablation energy source. The advantage of microwave energy is that it is much easier to control and safer to apply than direct current applications. Moreover, microwave energy is capable of generating substantially larger lesions than RF catheters, which greatly simplifies the actual ablation procedures. Accordingly, there are a number of catheters under development which utilize electro-magnetic energy in the microwave frequency range as the ablation energy source. By way of example, such systems are described in the U.S. Pat. No. 4,641,649 to Walinsky; U.S. Pat. No. 5,246,438 to Langberg; U.S. Pat. No. 5,405,346 to Grundy, et al.; and U.S. Pat. No. 5,314,466 to Stern, et al, each of which is incorporated herein by reference.
While these microwave ablation instruments may be employed to adequately ablate bio-tissue, most of the existing microwave ablation instruments contemplate the use of longitudinally extending helical antenna coils that direct the electromagnetic energy in a direction that is generally perpendicular to the longitudinal axis of the instrument although the fields created are not well constrained to the antenna itself. Although such instrument designs work well for a number of applications, it would also be desirable to provide microwave ablation instrument designs that are capable of effectively transmitting electromagnetic energy in other specific directions, such as a generally forward direction relative to the longitudinal axis of the instrument.
One such end-firing ablation instrument is disclosed in our U.S. Pat. No. 5,800,494. This instrument design incorporates a coaxial transmission line having an inner conductor, an outer conductor and a dielectric material medium disposed therebetween. At the distal end of the inner conductor is a spirally wound antenna coil adapted and oriented to emit an electromagnetic field in a direction distal to the longitudinal axis at the tip of the inner conductor.
Although this design provides functional distal firing capabilities, the width of the electromagnetic field is only about 2-4 mm wide. Accordingly, without substantially increasing the antenna dimensions and/or power output, use of the ablation instrument is limited to relatively small target regions.